WORKSHOP AGENDA ITEM 4: KNOWLEDGE OF IMPACTS OF BYCATCH

Impacts of shrimp trawl bycatch in Australia
Aubrey Harris, FAO regional office, Harare

Introduction

The Australian northern prawn fishery opened in 1963 as a result of a detailed survey undertaken by Ian Munro. The fish community structure in northern Australia is typical of the Indo-Pacific prawn grounds in that there is high species diversity and small sized fish species dominate the fish population. The fishery catches between 10 000 to 12 000 tonnes of prawns annually. The tiger prawns are fished during the night while the night fishing primarily targets the white banana prawn. High rates of discards are associated with the prawn fishery. Most of the finfishes are discarded.

Long-term ecological effects on bycatch

To elucidate the long term ecological effects on shrimp bycatch a study was carried out in the same area to compare the catches prior to the commencement of the trawl fishery and over the next 20 years after the commencement of the fishery. Initially, sampling was carried out over a two years period to get a measure of the annual variability. In the experimental design, sampling was carried out using identical prawn trawls (florida flyers), same month, same trawl position, same time (day/night) and the same species/taxa identification.

The abundance of 82 fish taxa (species, families, groups) was studied over the 20 years period. A total of 52 fish taxa representing 63 percent of the total fish taxa studied showed no change, 18 fish taxa (22 percent) showed a decrease in abundance, while 12 taxa (15 percent) showed an increase in abundance. There was no change in overall species richness.

The change in fish abundance may be due to the following factors:

• The increase or decrease in fish abundance could be explained by the position of the fish in the water column. Benthic dwelling fish decreased in abundance while the semi-demersal fish species increased in abundance.

• The changes could also be related to the fishing effect of prawn trawls and the use of discards as a source of food. The bottom trawl disturbed the demersal fish species more, leading to a decrease in abundance. The discards contributed significantly to the food source of the pelagic fishes leading to the increase in their abundances.

• Natural changes in sediment were implicated in the observed changes.

• A pronounced change in the diurnal abundance in the fish community may have contributed to the observed changes.

In this study, it was observed that long term impacts of trawling include:

• A change in fish composition over time. The change in composition may either have negative or positive implications. The values of the society involved in the utilization of the resource will determine the changes will be viewed.

• Species of particular concern, e.g. those that are long-lived, have low fecundity and of restricted distribution/habitat may be impacted more by trawling.

• There was no evidence of any loss in species richness, especially that of fish.

Long-term impacts of trawling on species of special concern

In the 1980s there were serious conflicts related to turtle mortality in the Australian northern prawn fishery. Conservationists claimed that the trawl fishery caught at least one to five turtles per trawl shot, a claim which was disputed by the trawl operators. To come up with factual information, a study was commissioned to gather information on the extent of turtle mortality caused by the trawl fishery. The information was gathered from scientific trawl surveys, from observers on board trawling vessels and from monitoring by trained fisher volunteers.

Results from the survey indicated that:

• One turtle is caught every three to four days of trawling. The trawl fishery accounted for only one to three percent of the turtle mortality depending on species.

• Catch rate of turtles depends on depth, time of the year and location of trawling.

• The condition on capture of the turtle was related to duration of trawl. There were no dead turtles in trawls of 30 minutes and in trawls of about three hours, 18 percent of the turtles caught were dead while 11 percent were in a comatose state.

• There were five species of turtles in the trawling ground.

• The trawl catch results were used to estimate the size of turtle populations and the information assisted in tracing the migration of some of the juveniles.

Conclusions

There was a debate whether the observed turtle mortality from trawling activities accounted for the observed decrease in some Australian turtle populations. It was however acknowledged that other activities account for significant proportion of the turtle mortality. These activities include the following:

• Traditional harvesting by indigenous people;
• Destruction of beaches, erosion, coastal development;
• Predation of eggs by foxes;
• Ingestion of floating debris;
• Boat strikes;
• Disease.

The study concluded that trawling for prawns was not the major source of mortality on turtles but the above activities contribute significantly to mortalities of turtles. However, there is need for increased efforts to reduce all forms of human induced mortality on turtles, particularly the loggerhead.

To reduce turtle mortality from the trawl industry, TEDs were adopted across the Northern prawn fishery in 2000. The adoption of TEDs in the trawl fishery was led by the industry itself. As a result, there is total compliance and the industry keeps documented history of the catch before and after use of TEDs to demonstrate their contribution to resolving the turtle issue.

KENYA

Ecological impacts of trawling on substrate and invertebrates in Kenya. A case study in Malindi-Ungwana Ba.
Esther Fondo, KMFRI, Mombasa

Any fishing gear that is towed over the seabed will disturb the sediment and the resident community to some degree. Fishing affects the seabed habitat worldwide on the continental shelf. These impacts are patchily distributed according to the spatial and temporal variation in fishing effort that results from trawling. As a consequence, the frequency and intensity of fishing disturbance varies among different habitat types. Different fishing methodologies vary in the degree to which they affect the seabed. Comparative studies of areas of the seabed that have experienced different levels of fishing activity demonstrate that chronic fishing disturbance leads to the removal of high biomass species that are composed mostly of emergent seabed organisms, thus lowering productivity. Conversely, scavengers and small-bodied organisms such as polychaete worms dominate heavily fished areas.

The resuspension, transport and subsequent deposition of sediment may affect the settlement and feeding of the biota in other areas. Sediment resuspension as a result of bottom trawling will have a variety of effects including: release of nutrients held in the sediment; exposure of anoxic layers; release of contaminants; increase in BOD; smothering of feeding and respiratory organs. The effects of trawling on the substrate and invertebrates may have the following implications:

• reduced topographic complexity therefore reducing habitat complexity and species diversity;

• removal of high biomass species leading to lowered productivity;

• removal of predators that control bio-engineering organisms;

• change in predator-prey relationships leading to shifts in food web structure;

• alteration of benthic community structure;

• small-bodied organisms such as polychaete worms dominate;

• resuspension of upper sediments leading to remobilization of contaminants and fine particulate matter;

• unstable sediment systems and destabilized chemical fluxes;

• resuspension, transport and subsequent deposition of sediment may affect the settlement and feeding of the biota in other areas.

Between June 2001 and June 2002, a study was carried out to gather baseline data on the impacts of commercial trawling on the benthic habitat of the Malindi-Ungwana Bay prawn trawling grounds. Preliminary results showed that the community structure was characterized by low species diversity and abundance. The benthic community was composed of various feeding groups which included predators (carnivorous), detritivores, scavengers, filter feeders, suckers and parasites. However, the dominant feeding groups were the predators (39 percent) and detritivores (34 percent). The low species diversity; dominance of the benthic community by predators, detritivores and the absence of suspension feeders may be an indication of changes in the benthic structure and reduced structural complexity in the bay due to the impacts of bottom trawling.

MADAGASCAR

Impacts of bycatch in Madagscar
Guy Rabarison (CNRE) and Olga Andriamiseza (Ministry of Fisheries), Antananarivo

Little is known on the impacts of bycatch on ecological or economical aspects. During the period 1980-1990, the only studies conducted on shrimp bycatch were those of CNRO (1988, 1989) and the FAO project (SWIOP, 1989). Effects of shrimp trawling on bycatch population (fish and invertebrates) are not documented in Madagascar. Information collected during the course of other projects shows that:

• In fishing grounds with high fishing effort levels, size reduction in fish species is noted.

• Traditional fishermen complain that their catch and catch rate have dramatically declined in trawling areas.

• Periodically, conflicts between shrimp trawlers and traditional fishermen using fixed traps (valakira) take place on the northwest coast.

• Some proliferation of sea urchins has been noted in the west part of Madagascar.

• There is very limited experience with turtle excluder device (TED) and other bycatch excluder devices. Some experimental work has been conducted by the shrimp society, but results are not available.

SOUTH AFRICA

Impacts of bycatch in South African prawn trawl fisheries
Sean Fennessy, Oceanographic Research Institute, Durban.

There are no historical data for the deep water trawl fishery, and collection of observer data only commenced in 2002, although there are data on retained bycatch which have been collected (via skipper’s logbooks) since 1985. Consequently it is not possible to determine whether there are ecological impacts by deep water trawlers because of the lack of detailed information. Based on the current knowledge of deep water bycatch composition (Fennessy and Groeneveld, 1997), there is no obvious impacts of the deep water prawn trawl fishery on other fisheries sectors.

In the shallow water fishery, there are user conflict impacts between trawlers and hook and line fishermen. The work by Fennessy (1994a) showed that trawlers were catching about 800 000 juvenile individuals of the sciaenid fish Argyrosomus thorpei per annum (mostly in January and February), while commercial hook and line fishermen were catching about 400 000 individuals of this species annually, albeit larger individuals. Per-recruit modelling demonstrated that the trawl catches were impacting on yield and spawning biomass of this species and, since about 1997, the shallow water Tugela Bank trawl fishery has been closed in January and February each year. The economic viability of the prawn fishery was also not good during these months, so it made economic sense not to trawl at this time and the trawlers had the possibility of trawling on the deep water grounds during these months. Good data is required to be able to undertake user conflict assessments of this type, specifically data on catch quantities, sizes of the relevant species caught by the different sectors, and information on age, growth and maturity of the relevant species. Currently, a similar assessment is underway for Otolithes ruber.

Regarding the potential for ecological impacts of shallow water trawling, there are historical data on catch composition from 1989 to 1993 and data is currently being collected by observers to enable comparisons to be made. Preliminary data from a relatively new trawl ground off St Lucia (Fennessy, 2002) indicates that trawlers cause substantial damage to organisms such as sea pens and echinoderms.